This application is entitled to the benefit of, and incorporates by reference, essential subject matter disclosed in Swedish Patent Application No. 0201982-6 filed on Jun. 24, 2002.
The present invention relates to a method of cleaning crankcase gas produced by an internal combustion engine from particles, solid and/or liquid, suspended therein, the crankcase gas being caused to rotate around a rotational axis by means of a centrifugal rotor which for its operation is journalled in a supporting device at two bearing places situated axially spaced from each other.
A method of cleaning crankcase gas in this way has been described in WO 01/36103. A centrifugal separator is used having a centrifugal rotor, that includes a stack of conical separation discs and that is journalled at both its ends on the respective sides of this stack. The centrifugal rotor is rotatable in a stationary housing, the crankcase gas to be cleaned being caused to flow through thin interspaces between the separation discs from a central chamber in the rotor radially outwardly to a chamber surrounding the rotor in the stationary housing. A motor which may be of any suitable kind is coupled to the centrifugal rotor at one of the ends of the latter for rotation thereof in the stationary housing.
In recent years centrifugal separators of a relatively advanced kind, e.g. the aforementioned centrifugal separator, have been suggested for cleaning of crankcase gas. Centrifugal separators of this kind are relatively expensive to produce, which necessitates that a device for driving such a centrifugal separator must be made very cheap. One starting point for the driving of a centrifugal separator of this kind has been, therefore, that an already existing source of energy, that is continuously maintained by the combustion engine giving off the crankcase gas to be cleaned, should be usable in the easiest possible way for the driving. Thus, it has been suggested that a mechanical coupling should be established between one of the motor shafts, e.g. the crank shaft or the cam shaft, and the centrifugal rotor (see for instance U.S. Pat. No. 5,954,035). Another suggestion has been that an electrical motor should be used (see for instance WO 01/36103). A further suggestion has been that a pressure fluid generated by the combustion engine, e.g. part of the pressurized lubricating oil circulating in the motor, should be used for driving the centrifugal rotor through a simple turbine device (see for instance WO 99/56883).
In addition to the cost for the production of a driving device for the centrifugal rotor also other aspects on the choice of a driving device have to be considered. For instance, it has to be considered how driving energy may be transferred in the easiest and cheapest way to the centrifugal rotor. For these reasons, among others, it is today still unclear which type of driving device will be accepted in the future for a centrifugal rotor for the purpose here concerned, particularly in connection with combustion engines for propelling vehicles.
The object of the present invention is, in the first place, to provide a new method of cleaning crankcase gas by means of a centrifugal rotor, which method can give equipment as inexpensive as possible for the driving of the centrifugal rotor, including necessary devices for the transfer of driving energy from a source of energy to the centrifugal rotor, and which method can give a freedom as large as possible for the location of the centrifugal rotor in relation to the combustion engine.
This object can be achieved according to the invention by bringing a centrifugal rotor of the initially defined kind into rotation by using part of it also as a rotor of an electrical motor having also a stator, the stator being kept non-rotating and charged with electric current for rotation of the centrifugal rotor, while the centrifugal rotor is kept journalled during its rotation only at said two bearing places.
In accordance with the invention an electrical motor is used, which does not have separate bearings for the journalling of the rotor of the motor. The already present and necessary bearings for the centrifugal rotor are used for this journalling. Since the centrifugal rotor is intended for treatment of a gas and, therefore, may be made substantially completely of a very light material, the electrical driving motor may be made relatively small and, thereby, also its rotor may be made very light. For these reasons the combined centrifugal and motor rotor may be made very light and, thus, the cost for its journalling may be reduced to a minimum.
When the method according to the invention is practiced, the centrifugal rotor need not necessarily be formed so that gas to be cleaned has to flow through channels within the centrifugal rotor. Instead, the centrifugal rotor may be arranged to bring into rotation, within a stationary housing, gas that flows in a thin layer axially through an annular space formed between the centrifugal rotor and the stationary housing.
At each one of said two bearing places a bearing should be present for taking up radial forces during rotation of the centrifugal rotor. In case the centrifugal rotor is arranged for rotation around a vertical rotational axis, there may be present at one of the bearing places also a bearing for taking up axial forces. Possibly, an angular contact bearing may be used, adapted to take up radial as well axial forces.
The invention also concerns a gas cleaning separator, by means of which for instance the above defined method of cleaning crankcase gas may be performed. A separator of this kind includes a non-rotatable supporting device, a centrifugal rotor and an electrical motor for rotation of the centrifugal rotor around a preferably vertical rotational axis, the centrifugal rotor being arranged to be journalled in the supporting device through bearings situated only at two bearing places axially spaced from each other and having a rotor body with a separating device, that forms several passages through the rotor body for through flow of the gas to be cleaned. The separator is characterized in that the electrical motor includes a stator that is connected with said supporting device, and a rotor that is constituted by part of said centrifugal rotor and is journalled relative to the stator only in said bearings.
Said separating device may be of different kinds. The patent DE 48615 discloses some examples of such a device. Thus, the separation device may include several discs placed in a ring around the rotational axis of the centrifugal rotor, each disc extending both axially and from a small to a larger distance from said rotational axis in a direction crossing several radii starting from the rotational axis. The separation device alternatively may include one single disc that is winded spirally around said rotational axis in a way such that the gas to be cleaned may flow axially through a channel that, seen in a section perpendicular to the rotational axis, extends like a spiral. It is also possible to fill out a separation space in the centrifugal rotor with fibres, which between themselves form said passages through the rotor body. Preferably, however, the separation device includes a stack of conical separation discs arranged coaxially with said rotational axis.
Said supporting device is preferably constituted by part of a stationary frame, with which it is immovably connected. However, if this would prove suitable, the whole supporting device, may be turnably suspended from a stationary frame, so that it is free to perform a pendulum movement relative to the frame.
The journalling of the centrifugal rotor in the supporting device may be of different kinds. Thus, the two bearing places may be situated axially on one side of the rotor body. In a preferred embodiment of the invention the rotor body is arranged axially between the two bearing places, however. In this case the rotor of the electrical motor is preferably situated on one side and the rotor body situated on the other side of one of the bearing places.
If desired, one part of said rotor body may constitute the aforementioned part of the centrifugal rotor, that is used as a rotor in the electrical motor. The stator of the motor then may surround or be surrounded by this part of the rotor body at a larger or smaller radius, for instance at a radius half as big as the biggest radius of the rotor body. In a particular embodiment the main part of the rotor body may be situated between the two bearing places, whereas part of the rotor body may surround and extend axially past one bearing place and support the rotor of the electrical motor in an area that is not situated at the same side of this bearing place as the main part of the rotor body.
The electrical motor as defined above may principally be of any suitable kind, for instance a direct-current motor or an alternate-current motor (synchronous motor or asynchronous motor). Preferably, a synchronous motor is used having a rotor that includes permanent magnets.